emerge can't install sci-misc/mendeleydesktop. as Mendeleydesktop for Linux is now mendeleydesktop-1.19.4-linux-x86_64.tar. sci-misc/mendeleydesktop-1.19.3 is what is in Gentoo tree, however, mendeleydesktop-1.19.3-linux-x86_64.tar is no longer available from Mendeley. Please update ebuild to reflect new version of mendeleydesktop-1.19.4. thanks in advance!
mendeleydesktop 1.19.3 is available here: https://desktop-download.mendeley.com/download/linux/mendeleydesktop-1.19.3-linux-x86_64.tar.bz2 Gentoo is often a version behind the release version of mendeley. You can find the older versions by identifying the download link for the current version and making sensible changes. The files continue to be hosted for some time even after the links are removed, you just need to be able to guess the download URL.
This appears to have been bumped in commit 4046f78ef0f7ac12cff0d4dbe0567f2585bc8925.